Exposing the myth of true position

Exposing the myth of true position

Clark, Richard

It’s a scenario that is playing itself out today in shops all over the country. A machinist brings a part in for inspection after CNC milling. The true position tolerance listed on the print (0.0015”) is viewed as “wide open.” After CMM layout inspection, the positions of the holes are found to be twice what is allowed according to print. The usual questions are asked: How far are they off? Is my X or Y axis position causing the problem? If this sounds remotely familiar, then you have experienced it also.

The myth of true position begins with what seem like common sense assumptions. If the true position tolerance is listed as 0.0015”, then we have close to that value to work with. In reality, you can rarely even use half of the value and still be within the print allowance.

Exposing the myth begins with understanding the geometries that define true position. Using the tolerance of 0.0015”, in the above example and a nominal position of X 5.0000, Y 5.0000, we can construct a point at the nominal position. (Note: For simplicity we will not factor in any bonus tolerance from hole diameter at MMC or LMC.)

Now we create a geometrical perfect circle, having a diameter equal to the true position tolerance, with the nominal position as its center point.

We have now established our boundary and target. For our hole to be within the 0.0015” true position tolerance, the eenter point must lie within this created circular boundary. We can now plug in some fictional center points to disprove some of the common sense assumptions.

Maybe if we only used what seems like half of the tolerance and the center point of our hole was located at X 5.0007”, Y 5.0007”, we’d be there.

This 0.0015” tolerance just keeps getting smaller. So how much could we be “off” in each axis and still be within tolerance? The answer: 0.00053”. That’s not a typo, 5 tenths and 30 millionths.

That is the reality of true position, we only have a little over a third of the listed tolerance to use in each axial direction.

This is a good target to use but keep in mind it’s not cast in stone. We could have scenarios where more than one third is used in one direction if the other direction used much less than one third. A best case example of this would be if our center point were located at X 5.0000”, Y 5.0007” or X 5.0007”, Y 5.0000”.

GD&T examples such as this serve as models to improve the communication between a shop’s machining and inspection areas.

After all, the cost of a good, used CNC machining center: $70,000

Cost of today’s high end inspection equipment: $30,000 $80,000

Having your machinists and inspectors on exactly the same page… Priceless

Richard Clark works as a Metrologist in an industrial setting. Subscribe to his free e-newsletter “Measurement on the real shop floor” by e-mailing feedback to rcmetrology@yahoo.com

Copyright Nelson Publishing Oct 2003

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